This application claims benefit of priority under 35USC xc2xa7 119 to Japanese Patent Application No. 2001-58916, filed on Mar. 2, 2001, and No. 2002-46618, filed on Feb. 22, 2002, the entire contents of which are incorporated by reference herein.
1. Field of the Invention
The present invention relates to a method for calibrating measuring machines and more particularly to a method and apparatus suitable for calibrating surface texture measuring machines such as three-dimensional measuring machines and the like.
2. Description of the Related Art
A three-dimensional measuring machine for measuring a three-dimensional shape of a work; a contour measuring machine and an image measuring machine for measuring a two-dimensional contour; a roundness measuring machine for measuring roundness; and a surface roughness measuring machine for measuring waviness and roughness of a work surface have been known as surface texture measuring machines that can be employed to measure surface shapes of works, such as contour, roughness and waviness. These machines are generally equipped with a guide mechanism for moving a contact or non-contact sensor relative to a work in one- or multi-axial arrangement.
These guide mechanisms commonly include a guide, an feed screw and a nut for mating with the screw to move a slider that is coupled to the nut. A linear scale is employed, for example, to measure a movement of the slider. There is another type of guide mechanism that is not always equipped with an feed screw but consists only of a guide and a slider. This guide mechanism employs a linear scale to read an amount of displacement of the slider manually moved. The slider is commonly provided with a sensor such as a touch probe and a CCD camera attached thereon.
Environmental pollution, environmental variation-related deformations and other errors are inevitably caused in these guide mechanisms. As a result, the slider can not move correctly and may give errors in data measured from a work by the sensor located on the slider.
For example, of the above guide mechanisms, in a straight guide mechanism designed for the purpose of straight movement, various errors can be considered: a straightness error in a vertical plane; a straightness error in a horizontal plane; a pitching error; a rolling error; a yawing error; and an indication error on the linear scale itself.
Of the surface texture measuring machines, as a three-dimensional Coordinate Measuring Machine (hereinafter referred to as CMM) has a structure that includes three sets of such straight guide mechanisms intersecting at right angles with each other, orthogonal errors occur between the straight guide mechanisms additionally. Therefore, at least 21 types of geometrical deviations in total may possibly occur in such the CMM.
As a result, a great effort is required disadvantageously in an operation to strictly calibrate such the surface texture measuring machines.
For example, a measuring machine designed for the purpose of calibrating a geometrical deviation of the CMM is currently limited from the viewpoint of the variety of measurement methods while it has been employed long in history. In many cases, the mainstream is a measurement instrument for mono-functionally detecting a geometrical deviation, for example, a laser interferometer and an electrical level. To manage uncertainty in measurement using the measuring machines, it is required to handle the machine and perform alignment prior to every measurement by an operation-learned operator. As a result, it is required to perform calibration by a skilled worker spending many hours, resulting in a high-cost, labor-intensive work step that can not expect a saving in labor. On the other hand, when the geometric accuracy by the current CMM is standardized within its operable range, it has already reached several ppm. Thus, it is difficult to realize such a calibration method that can be satisfied from the viewpoint of uncertainty in view of simply trying automation.
Reflecting the recent high concerns on traceability and uncertainty in calibration, a trend can be found in an offer of a geometric calibration to the user for an appropriate market price and quality. In such the case, it can not be expected to ensure a calibrating operator with extremely high techniques. Even if it can be expected, the user""s satisfaction from the viewpoint of cost remains low. More importantly, the geometric calibration in the market aims at an additional calibration, which is performed to a measuring machine already calibrated generally by the maker using some method, to issue an official certificate of calibration on uncertainty in calibration. Therefore, in the case of the CMM, it is not required to measure a measurement space including everything. In addition, it is possible to evaluate at a considerably long interval between measurement points. With this regard, it has a characteristic of the sampling test.
To the contrary, the calibration in the process of manufacturing CMMs has a different property from that in the market. First, as the object is a CMM that is not calibrated previously in history, it is required to locate measurement points that can cover the whole measurement space at a necessarily and sufficiently fine interval. This corresponds to a 100% and full-function inspection. In addition, a premise lies in compensating the geometric deviation of CMM using the calibration result. Therefore, it is required to adopt a calibration method that provides a calibrated value as the geometric deviation kinematically described usable for compensation of precision. Due to such the property, the dependency on the learned worker is particularly higher compared to the commercial calibration laboratory, presenting a high barrier against saving in labor.
The present invention has been made to solve the above disadvantages and accordingly has an object to provide a calibration method capable of increasing precision of spherical parameter assumption to improve precision of calibration.
To achieve the above object, the present invention is provided with a first calibration method, comprising the steps of: positioning a reference device having a sphere within a measurement space by an object three-dimensional measuring machine having a spherical probe; contacting the spherical probe with six or more measurement points uniformly distributed on the spherical surface of the sphere to measure central coordinates of the sphere of the reference device by the object three-dimensional measuring machine; and calibrating the object three-dimensional measuring machine based on the central coordinates obtained.
The present invention is provided with a second calibration method, comprising the steps of: positioning a reference three-dimensional measuring machine having a first probing system previously calibrated and an object three-dimensional measuring machine having a second probing system to be calibrated in such a manner that a measurement space by the three-dimensional reference measuring machine is superimposed on a measurement space by the object three-dimensional measuring machine, locating a spherical probe on one of the first and second probing systems and locating a reference device having a sphere on the other of the first and second probing systems; contacting the spherical probe with six or more measurement points uniformly distributed on the spherical surface of the sphere of the reference device to acquire first measurement values by the reference three-dimensional measuring machine and second measurement values by the object three-dimensional measuring machine; and calibrating the object three-dimensional measuring machine based on the first and second measurement values.
According to the present invention, six or more points uniformly distributed on the spherical surface of the sphere provided on the reference device are employed as measurement points to assume parameters of the sphere. Therefore, it is possible to obtain highly reliable data with improved xe2x80x9cuncertainty in measurementxe2x80x9d that are isotropic in X, Y and Z directions and not correlated with each other.